Linear reading thermometer

ABSTRACT

A silicon diode is used in a bridge circuit designed to provide linear changes in current through a meter in the bridge in response to changes in temperature on the diode. Variations from linear response in individual diode resistance characteristics as a function of temperature are compensated by a resistor in series with the diode and a second resistor in parallel with the diode, with these resistors being selected so that the resistance versus temperature characteristic of the entire circuit is fixed, thereby providing linear response and allowing interchangeability of diode temperature measuring circuits with a single bridge. All legs of the bridge circuit include resistances of the same order of magnitude to provide maximum sensitivity of the bridge to temperature measurements. In one embodiment one leg of the bridge is provided with a plurality of resistors, one of which is connected at a time in the bridge circuit to maintain the bridge near a balance condition through the entire temperature range of the instrument. In another embodiment, fixed resistors are used in each leg with readout being provided by a digital volt meter.

United States Patent Treharne et al.

[ Mar. 27, 1973 Riley; Eddie R. Thomas, Yellow Springs, Ohio [73]Assignee:

Kettering Scientific Research Inc.,

Dayton, Ohio Filed:

Nov. 17, 1971 Appl. No.: 199,479

Related US. Application Data Continuation-impart of Ser. No. 29,197,April 16,

1970, abandoned.

Int. Cl.

US. Cl. ..73/362 SC, 73/362 AR, 307/310,

..-................G0lk 7/22 Field of Search ....73/362 AR, 362 SC;307/310;

323/75 F, 75 H, 75 N References Cited UNITED STATES PATENTS POWER2,653,308 9/1953 Allen ..73/362 AR X 4/1969 Lightner ..73/362 SC PrimaryExaminer-Louis R. Prince Assistant Examiner-Frederick ShoonAttomey--Lawrence B. Biebel et a1.

[57] ABSTRACT A silicon diode is used in a bridge circuit designed toprovide linear changes in current through a meter in the bridge inresponse to changes in temperature on the diode' Variations from linearresponse in individual diode resistance characteristics as a function oftemperature are compensated by a resistor in series with the diode and asecond resistor in parallel with the diode, with these resistors beingselected so that the resistance versus temperature characteristic of theentire circuit is fixed, thereby providing linear response and allowinginterchangeability of diode temperature measuring circuits with a singlebridge. All legs of the bridge circuit include resistances of the sameorder of magnitude to provide maximum sensitivity of the bridge totemperature measurements. In one embodiment one leg of the bridge isprovided with a plurality of resistors, one of which is connected at atime in the bridge circuit to maintain the bridge near a balancecondition through the entire temperature range of the instrument. Inanother embodiment, fixed resistors are used in each leg with readoutbeing provided by a digital volt meter.

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TEMPERATURE 'C PATENTEDuAazmYa SHEET 3 BF 3 FIG 6 I 9 5 a I! 54 55 BEIII Powan FIG-8 f THERMISTOR FIG-8f moms I I I FIG-1O THERMISTOR I DIODE(FORWARD RESISTANCE) LINEAR READING THERMOMETER RELATED APPLICATION Thisapplication is a continuation-in-part of copending application Ser. No.29,197, filed Apr. 16, 1970, and now abandoned.

BACKGROUND OF THE INVENTION A semi-conductor diode will exhibit a changein resistance which is inversely proportional to a change in thetemperature to which the diode is subjected. This change in resistanceto temperature is nonlinear, and it is therefore necessary to employspecial techniques if a linear change in electrical quantity, eithervoltage or current, with respect to a change in the diode temperature isdesired.

One technique for obtaining a linear electrical output is shown in US.Pat. No. 3,092,998 where a germanium diode is included in one leg of abridge circuit and wherein the current through the diode is maintainedat a substantially constant magnitude by including a relatively highresistance in another leg of the bridge. Since the resistance values inthe legs of the bridge differ by at least an order of magnitude, maximumsensitivity of the bridge over wide temperature ranges is not obtained.Moreover, no means for compensating for individual diode characteristicsto provide interchangeable sensors is described.

Another technique, shown in US. Pat. No. 3,330,158, uses a matchedbridge circuit wherein each of the legs of the bridge is composed of asimilar diode, each having substantially identical temperatures versusvoltage characteristics, with two of the diagonally pposite diodes beingpositioned in the environment the temperature of which is to bemeasured. There is no provision, however, for compensating the bridgefor a wide variation in diode resistance as the temperature of the diodechanges, and again maximum sensitivity is not obtained.

Thermistors have also been used as temperature responsive devices,examples of which are shown in the following US. Pat. Nos. 2,271,975;2,764,731; 2,971,379; 2,938,385 and 3,036,464. A thermistor, however,does not react to changes in temperature in the same manner as a diode.Therefore, the techniques used to linearize the output of thermistortype temperature sensors are not the same as those used in the presentinvention to linearize the output of the diode. A diode is characterizedby a nonlinear voltage-current relationship at a constant temperaturewhile a thermistor possesses a linear or ohms law voltage-currentrelationship at a constant temperature. In the present invention, theunique characteristics of a diode with changes in temperature are usedto produce a linear reading thermometer which may read temperatures overan extremely wide range, as from 200 to +200C.

SUMMARY OF THE INVENTION This invention relates to an improved linearreading electronic thermometer and more particularly to a linear readingthermometer employing a single semiconductor diode, preferably a silicondiode, in one leg of a bridge circuit, all of the legs of which are ofthe same order of magnitude as the resistance of the diode at thetemperature to which it is subjected.

A constant voltage source is connected across diagonally opposedterminals of a bridge circuit having legs of approximately equalresistance with a meter being connected to the remaining diagonallyopposed terminals to measure the current flow in the bridge circuit. Thediode temperature measuring element is connected as the resistance inone leg of the bridge circuit and in one embodiment, a plurality ofresistors, one of which is selected by a switch, are found in anotherleg of the bridge circuit so that the temperature range to which thediode may be subjected may be expanded and displayed at maximumsensitivity on the meter scale.

The invention also contemplates the use of interchangeable diode unitswith a single instrument. Interchangeability as well as linearity isprovided by including in the diode circuit a pair of resistors, one ofwhich is in series with the diode and the other of which is in parallelwith the diode. The resistance values of these resistors are so selectedthat the entire circuit will obtain predetermined resistance values forspecified temperatures.

In the preferred embodiments of the present invention, the currentthrough the diode is allowed to vary by as much as fifty percent withchanges in temperature, and due to the voltage-current characteristic ofa diode, a linearized output may be obtained. This is not true of any ofthe prior art devices, particularly U.S. Pat. No. 3,092,998 where thecurrent through a germanium diode is held at a nearly constant value.This is done by unbalancing the bridge circuit in which the diode isconnected, and therefore linearity and maximum sensitivity are notobtained.

The nonlinearity in the resistance versus temperature response of thediode used in this invention is partially compensated by thecharacteristics of a substantially balanced bridge circuit with finallinearization being accomplished by means of series and shunt resistorsso that a linear change in bridge current results from linear changes indiode temperature. By providing a matched bridge circuit, that is acircuit wherein each leg contains resistances of approximately equalvalue, maximum sensitivity to temperature changes is obtained. Accuracyin the order of i%C is consistently obtainable over a temperature rangeof at least 200 to +200C.

Accordingly, it is an object of this invention to provide an improvedlinear reading electronic thermometer employing a diode temperaturesensing element as one resistance in a bridge circuit includingresistances of approximately equal value; to provide an improved linearreading thermometer of the type described wherein the diode temperaturesensing element is corrected for individual variations in dioderesistance so that the sensors can be interchangeable with severalinstruments, or wherein the instrument may accept several differentdiode elements; and to provide a linear reading thermometer using asilicon diode and a digital readout device.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of anelectronic thermometer employing the diode element and bridge circuit ofthis invention;

FIG. 2 is an electrical schematic diagram showing a bridge circuitdesigned according to the principles of this invention;

FIG. 3 is a curve showing the relationship of the bridge output currentto changes in the resistance of one leg of the bridge;

FIG. 4 is a curve showing resistance of the temperature measuring diodeas a function of temperature;

FIG. 5 is a curve showing the current through the meter of FIG. 2 as afunction of the temperature of the diode;

- FIG. 6 is a perspective view of another embodiment of the inventionemploying a digital readout;

FIG. 7 is an electrical schematic diagram showing a bridge circuit foruse with a digital readout device;

FIG. 8 is a curve showing the linear or ohm s law current-voltagerelationship of a thermistor;

FIG. 9 is a curve showing the nonlinear current-voltage relationship ofa diode; and

FIG. 10 includes a pair of curves showing the resistance-temperaturerelationship of both thermistors and diodes.

DESCRIPTION OF THE PREFERRED EMBODIMENT cabinet 10. A meter pointer 21may be viewed against a meter face including several scales, eachoverlapping slightly, and each calibrated in degrees centigrade. Thedivisions on the scale are linearly spaced since the cur rent throughthe meter will change in a linear relation to a change in temperature ofthe diode 15.

The temperature range, and therefore the scale to be read, is selectedby a selector switch 25 mounted on a generally horizontal portion of thecabinet 10. This selector switch, as will be explained, includes fourdecks or switch elements to select appropriate resistances for thebridge circuit. The selector switch includes an off position whichdisconnects the instrument from an external source of power, acalibrating position which is used to calibrate the instrument prior touse, and four temperature ranges. The maximum temperature in each rangeis printed adjacent the switch. In the calibrating position, the needle21 of the meter is positioned to a predetermined calibrating mark byadjusting a calibrating potentiometer R1 which is provided with ascrewdriver slot since only infrequent calibration of the instrument isrequired.

Referring now to the electrical schematic diagram in FIG. 2, theinstrument is connected to a source of l 15 volt A.C. power by a plug30. One terminal of this plug is connected through deck 25a of selectorswitch 25. In the first position of the switch, the instrument isdisconnected from the source of power, however in the remaining fivepositions of the switch, the power source is connected through fuse 31to a constant voltage power supply 25. An Even Volt, Model 402-21, powersupply has been found useful for this purpose. The out put of this powersupply, which is 1.345 volt DC. at four milliamps, is applied todiagonally opposed terminals 36 and 37 of a bridge circuit showngenerally at 40. The calibrating potentiometer R1 is connected in serieswith one of the output terminals of the power supply 35 and terminal 37.

The bridge circuit 40 includes four legs each having approximately equalresistance. The two upper legs include resistors R2 and R3. One of thetwo lower legs, shown to the right in FIG. '2, includes resistors R4through R7, one of which is selected by deck 25b of the selector switch25.

The other lower leg, shown at the left, includes either resistor R8, orthe diode temperature measuring device 15, according to the position ofdeck 25c of the selector switch. The selector switch 25 is shown in theoff position, and when moved to the calibrating or second position,resistor R8 will be connected in the bridge circuit. In the remainingpositions of the selector switch, the

diode temperature measuring device 15 will be connected in the circuit.

The meter 20 for measuring the current in the bridge circuit isconnected to the remaining diagonally opposed terminals 41 and 42 of thebridge circuit. In the preferred embodiment, meter 20 is a twentymicroampere movement and is connected in series with an adjustingpotentiometer R9 and a fixed resistor R10. Connected in parallel withresistors R9 and R10 are resistors R11, R12 and R13, one of which isselected by deck 25d of the selector switch.

In order to match the characteristics of a silicon diode to the tocircuit to provide linear response at the bridge output, it has beenfound necessary to include series and parallel resistors, sinceindividual diodes may vary in their resistance at a given temperature.It has also been found desirable to provide an instrument whereina'plurality of diode temperature measuring devices can beinterchangeably connected to the instrument where, for example, one suchdevice becomes damaged or where a plurality of temperature measurementsare to be made utilizing a single display instrument. To assist inproviding linear response to compensate for individual diodecharacteristics, a series resistor Rs and a parallel resistor Rp areincluded in a plug in assembly with the diode 15. Thus, the diode 15 Iand resistors Rs and Rp are contained within a separate assembly whichmay be connected to the bridge circuit 40 by means of a plug 45.

The following table provides the resistance and tolerance values of theresistors used in the embodiments of FIG. 2:

TABLE] R1 ohm adjustable R2 1000 ohm 0.1%

R3 1000 ohm 0.1%

R6 789 ohm 0.1%

R7 640 ohm 0.1%

R8 800 ohm 10% Each leg of the bridge circuit includes resistances ofapproximately the same order of magnitude, about 1000 ohms. In FIG. 3, atypical relationship between the current through the meter 20 and theresistance of 1 one of the legs of the bridge is shown by curve 50. Thisrelationship is nonlinear. In FIG. 4, a typical resistance curve ofdiode 15 with respect to temperature is shown by the curve 55. This isalso a nonlinear relation. FIGS. 8 and 9 show the current-voltagerelationship of thermistors and diodes, respectively. The thermistorpossesses a linear or ohms law current-voltage relationship at constanttemperature while a diode is characterized by a nonlinearcurrent-voltage relationship. These curves illustrate one importantdistinguishing characteristic between diodes and thermistors. FIG. 10clearly shows the differences between the resistance-temperaturerelationship of thermistors and diodes as a second distinguishingcharacteristic. In the present invention, by careful selection of thebridge response curve as related to the resistance-temperature curve' ofthe diode 15, it is possible to make use of the unique characteristicsof a diode and obtain a device wherein the current through the meter 20varies as a nearly linear function of temperature, as shown by the curve65 in FIG. 5. Resistors Rs and Rp are used to provide finallinearization as shown by curve 60.

Since the diode varies in resistance inversely to temperature, the valueof the resistance in the opposite leg of the bridge is also varied tobalance the bridge substantially, thus operating the bridge at itsmaximum sensitivity throughout the temperature range for which theinstrument is designed. Thus for the lowest temperature range, resistorR4 is l 147 ohms which is the resistance of the diode assembly,including resistors Rs and Rp, at 80C. Similarly, resistances R5, R6 andR7 approximately equal the resistance of the diode assembly at 30C, +Cand +70C, respectively.

Referring again to FIG. 5, the curve 65 shown in dashed lines representsthe response of the instrument to changes in temperature of a diodewhich is connected to the circuit without the compensating resistors Rsand Rp. It will be seen that this curve is slightly nonlinear anddisplaced from curve 60. The amount of nonlinearity and displacementwill vary from diode to diode due to individual resistancecharacteristics, and in order to make the probe assembly linear andinterchangeable, the compensating resistors Rs and Rp are carefullyselected to bring the response curve of the instrument in line with thecurve 60.

Resistors Rs and Rp are selected in the following manner. The diode 15is first subjected to a temperature of 100C (boiling water) and theseries resistor Rs initially adjusted to provide a 100 reading on themeter 20. The diode is then cooled to 0C (ice and water mixture) and theparallel resistor Rp is adjusted to give a 0 reading on the meter 20.This procedure is repeated several times since there is an interactionbetween resistors Rs and Rp. Finally, the probe is subjected to a l95.8Ctemperature (liquid nitrogen), and then to +l80C (a calibrated oven) andthe meter readings checked. Only the diode 15 is subject to thetemperature to be measured since the resistors Rs and Rp are mountednear the plug 45 which attaches to a receptacle in the back of theinstrument housing.

Reference is now made to FIGS. 6 and 7 which show another embodiment ofthe invention wherein a digital readout device is used to display thetemperature of the diode. In this embodiment, the thermometer includes aO cabinet 50 having a digital readout device 52 to display clearly thetemperature in degrees Centigrade. On the front of the cabinet, andavailable to the operator, is an on-off switch 53 and a receptacle 54which receives a plug in cable 55, one end of which is attached to atemperature sensing probe.

In this embodiment, no range switch is necessary since the digitalreadout device will display the temperature throughout the entire range'(200 to +200 C) of the instrument.

Referring now to FIG. 7, the instrument is connected .to a source ofpower by an electrical cable 57. A power supply, shown generally at 60,includes a power transformer 61, which, in the preferred embodiment is aStancor Model T-A36l transformer. The secondary winding of thistransformer is connected to a full wave diode bridge rectifier 63. The12 volt AC output of the bridge rectifier 63 is applied to a filtercapacitor C1 and to an integrated circuit voltage regulator 65. Thevoltage regulator shown'herein is an RCA type CA305 5 silicon monolithicintegrated circuit device. The resistors and capacitors associated withthe voltage regulator are selected to give the voltage output of thepower supply of from 1.8 to 2.5 volts DC, with the actual voltage beingdetermined by potentiometer R14. I

The output of the power supply circuit 60 is applied across a bridgecircuit 70. Since diodes, unlike thermistors, are polarity sensitive,the polarity of the voltage applied to the bridge must be as indicatedfor proper operation of the diode. The diode must be forward biased andthe bipolar meter polarity also must be as shown. The bridge circuit hasfour legs, each with approximately equal resistances. The two upper legsincludes resistors R20 and R21. One of the two lower legs, shown at theright in FIG. 7, includes resistor R22. The remaining'legs includes thediode sensing device and two calibrating and linearizing resistors R23and R24.

A digital volt meter is connected in parallel with an adjustableresistor R25 across the bridge 70. The digital volt meter may be aWeston Model 1292 bipolar, digital volt meter with a range of i200millivolts. The bridge is so designed that a change of one degreecentigrade at the probe 75 is represented by a change of one millivoltacross the meter terminals of the bridge.

The diode 75 is located at the end of the cable 55 while resistors R23and R24 are located at the instrument end of the cable immediatelyadjacent to the connection or receptacle 54 on the instrument housing.Therefore, only the diode 75 is subjected to the tem peratures to bemeasured.

The value of resistors R22, R23, R24 and R25 are determined byexperiment in the following manner. R22 and R25 are set to approximately1000 ohms each. Diode 75 is connected to the bridge without R23 and R24.The power supply 60 is adjusted so that it supplies an output of 2.4volts. Liquid nitrogen at --l95.8C, an ice bath at C, and boiling waterat +100C are used as the three primary calibration points.

, The diode probe 75 is placed into an ice bath and resistor R22 isadjusted until the digital volt meter reads 0C. The diode -75 is thenremoved from the ice bath and inserted into liquid nitrogen. ResistorR25 is then adjusted until the digital volt meter reads 200C. Diode 75is then inserted into boiling water and the reading on the digital voltmeter observed. This reading, for the time being, should be +105C, 13C.If it is not within this range, then the steps outlined above arerepeated.

Resistors R23 and R24 are then installed with R24 being adjustedinitially to a zero ohm value and R23 adjusted to approximately 5000ohms. Diode 75 is placed again into liquid nitrogen and R23 adjusteduntil the digital volt meter reads 196C, i1C. The diode 75 is thenplaced in boiling water and R24 adjusted until the digital volt meterreads from 99 to 101C. The diode is then placed in an ice bath at 0C andthe reading on the digital volt meter checked. All of the above stepsare repeated adjusting R23 finally for a -195.8C reading when the probeis in liquid nitrogen and adjusting R24 for a +100C, 105C reading whenthe probe is in boiling water.

The following table provides the resistances and capacitances of thecomponents identified in FIG. 7.

TABLE II C1 5000 afd, 25V R19 22 ohms C2 20 pf R20 1 K ohms C3 2 ufd R211 K ohms C4 2 p.fd R22* approximately 643 ohms C5 5 pfd, 50V R23*approximately 2500 ohms R14 5 K ohms R24* approximately 45 ohms R15 1 Kohms R25* K ohms potentiometer R16 2700 ohms adjusted according to theabove instructions R17 5.6 ohms R18 1 K ohms In the embodiment of theinvention shown in FIGS. 6 and 7, the current flow across the diagonallyopposed terminals of the bridge circuit is detected by volt meter 80connected in parallel with resistor R25. This'differs slightly from theembodiment of FIGS. 1 and 2 where the meter 20 measures currentdirectly. The bridge 70 is adjusted to be in balance when the diode 75is exposed to 0C, and therefore no current flows through resistor R25.This means that resistor R22 will have the same resistance as the probe,including diode 75 and resistors R23 and R24 when the diode is exposedto 0C. For each change in temperature of the diode of 1C, the currentthrough R25 will cause the voltage thereacross to change by onemillivolt.

The instruments thus described provide a linear response in bridgecurrent to changes in temperature on a silicon diode and, since thebridge includes nearly balanced legs, maximum sensitivity over thedesired temperature range is achieved.

Instruments constructed according to the teachings of this invention arefound to be accurate within iWC over the entire operating range (200C to+200C) of the instrument described herein.

While the forms of apparatus herein described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to these precise forms of apparatus, and that changes maybe made therein without departing from the scope of the invention asdefined in the appended claims.

What is claimed is:

1. A linear reading electronic thermometer including a constant voltagesource;

a bridge circuit including four legs each having approximately equalresistances, said bridge circuit having diagonally opposed terminalsconnected to said voltage source;

means for measuring and displaying the current in said bridge circuit,said means connected .to the remaining diagonally opposed terminals ofsaid bridge circuit; and j a diode temperature measuring deviceconnected as the resistance in one leg of said bridge, said deviceincluding a silicon diode, the resistance of which varies inversely as afunction of temperature, a first resistor connected in parallel withsaid diode and a second resistor connected in series with said parallelconnection, the resistance of said first and second resistors beingadjusted to compensate for the individual characteristics of said diodeso that the diode temperature measuring device has atemperature-resistance characteristic which compensates for thenonlinear characteristics of said bridge circuit thereby causing currentthrough said measuring and displaying means to vary linearly withvariations in the temperature to which said diode is exposed.

2. The electronic thermometer of claim 1 wherein said diode measuringdevice is removably connected in said bridge circuit and wherein saidfirst and second resistors compensate for variations and nonlinearity inthe individual resistance characteristics of said diode thus allowinginterchangeability of diode measuring devices with said bridge circuit.

3. The electronic thermometer of claim 1 further including a calibratingresistor connected between said constant voltage source and said bridgecircuit.

4. The electronic thermometer of claim 1 wherein one of the legs of saidbridge circuit includes a plurality of resistors, one resistor beingconnected in said bridge circuit at a time to balance said bridgecircuit substantially thus to enable a linear change in current throughsaid measuring and display means as temperatures are measured by saiddiode over a wide range.

5. The electronic thermometer of claim 4 wherein the resistance of eachof said plurality of resistors equals the resistance of said diodetemperature measuring device at the lowest temperature of saiddiode in apredetermined range of temperatures.

6. The electronic thermometer of claim 1 further including a pluralityof resistors connected in series with said measuring and display means,said resistors being selected in accordance with the range 'oftemperature being measured to compensate for minor changes in currentthrough said measuring and display means for various temperature ranges.

7. The thermometer of claim 1 wherein said measuring and display meansincludes a meter connected across said remaining diagonally opposedterminals to measure the output current in said bridge circuit.

8. The thermometer of claim 1 wherein said measurdiode. ing anddisplaying means includes a resistor connected 9- The thermometer OfClaim 1 herein Said bridge across said remaining diagonally opposedtermi l d circuit is in balance when said diode is sensing a tem- I O adigital volt meter for displaying the voltage across P Ofo said resistoras a function of the temperature of said

1. A linear reading electronic thermometer including a constant voltagesource; a bridge circuit including four legs each having approximatelyequal resistances, said bridge circuit having diagonally opposedterminals connected to said voltage source; means for measuring anddisplaying the current in said bridge circuit, said means connected tothe remaining diagonally opposed terminals of said bridge circuit; and adiode temperature measuring device connected as the resistance in oneleg of said bridge, said device including a silicon diode, theresistance of which varies inversely as a function of temperature, afirst resistor connected in parallel with said diode and a secondresistor connected in series with said parallel connection, theresistance of said first and second resistors being adjusted tocompensate for the individual characteristics of said diode so that thediode temperature measuring device has a temperature-resistancecharacteristic which compensates for the nonlinear characteristics ofsaid bridge circuit thereby causing current through said measuring anddisplaying means to vary linearly with variations in the temperature towhich said diode is exposed.
 2. The electronic thermometer of claim 1wherein said diode measuring device is removably connected in saidbridge circuit and wherein said first and second resistors compensatefor variations and nonlinearity in the individual resistancecharacteristics of said diode thus allowing interchangeability of diodemeasuring devices with said bridge circuit.
 4. The electronicthermometer of claim 1 further including a calibrating resistorconnected between said constant voltage source and said bridge circuit.5. The electronic thermometer of claim 1 wherein one of the legs of saidbridge circuit includes a plurality of resistors, one resistor beingconnected in said bridge cIrcuit at a time to balance said bridgecircuit substantially thus to enable a linear change in current throughsaid measuring and display means as temperatures are measured by saiddiode over a wide range.
 6. The electronic thermometer of claim 4wherein the resistance of each of said plurality of resistors equals theresistance of said diode temperature measuring device at the lowesttemperature of said diode in a predetermined range of temperatures. 7.The electronic thermometer of claim 1 further including a plurality ofresistors connected in series with said measuring and display means,said resistors being selected in accordance with the range oftemperature being measured to compensate for minor changes in currentthrough said measuring and display means for various temperature ranges.8. The thermometer of claim 1 wherein said measuring and display meansincludes a meter connected across said remaining diagonally opposedterminals to measure the output current in said bridge circuit.
 8. Thethermometer of claim 1 wherein said measuring and displaying meansincludes a resistor connected across said remaining diagonally opposedterminals and a digital volt meter for displaying the voltage acrosssaid resistor as a function of the temperature of said diode.
 9. Thethermometer of claim 1 wherein said bridge circuit is in balance whensaid diode is sensing a temperature of 0*C.